Aluminum or aluminum alloy (hereinafter collectively referred to as "aluminum") is often surface treated to improve its hardness and resistance to corrosion, as well as to provide a substrate for depositing organic dyes or inorganic metal salts. Such treatments generally involve laying down a porous film or coating of adherent aluminum oxide on the aluminum surface, typically by direct current anodizing of the aluminum in an aqueous strong acid electrolyte, although other techniques and electrolytes are available. However, the aluminum oxide films because of their porosity remain somewhat vulnerable to corrosion, leaching of colorant or other deterioration; and in general must be "sealed" or the pores otherwise protected, for the films to be completely useful, particularly if employed, for example, in architectural uses.
Sealing has traditionally been carried out as a hydrothermal process in which the porous, essentially anhydrous aluminum oxide film is immersed in an aqueous bath maintained at a temperature at or near the boiling point, to result in the formation of hydrated aluminum oxide compounds such as boehmite. The formed hydrated compounds are believed to cause a constriction or blockage of the pores of the aluminum oxide film, producing the observed sealant effect.
A generally accepted measure of seal quality is "acid dissolution test" (ADT) weight loss, i.e. the weight loss resulting from exposure of the sealed aluminum oxide film to a dilute acid solution, typically at 100.degree. F. for 15 minutes. Most commercial applications require a "high quality" seal, by which is commonly meant seal characterized by an ADT weight loss of about 3 mg./in..sup.2 or less, and preferably about 2 mg./in..sup.2 or less.
Demineralized water at or near the boiling point has traditionally been used to make up sealing baths. Acceptable quality seal has also been obtained from ordinary tap water baths and at somewhat reduced temperatures, e.g., about 160.degree.-190.degree. F., by the addition of various additives to the bath, in particular, soluble salts of divalent metals, especially cobalt or nickel acetate.
However, a recognized limitation associated with aqueous sealant compositions or water per se as steam or hot water to seal anodized aluminum is the tendency for residual hydrated aluminum oxide crystals to deposit as "smudge" or "smut" on the aluminum oxide surface, often seriously detracting from its appearance. Chemical and mechanical desmutting after-treatments introduce an added expense and may themselves adversely affect seal quality.
Accordingly it has been a longstanding object in the art to prevent or minimize initial formation of smut during the sealing operation, without detracting from seal quality.
It has also become an important objective in the industry to reduce or eliminate potentially environmentally hazardous effluent from aluminum finishing processes, and it has been of particular interest to develop cobalt- and nickel-free aqueous sealant compositions for sealing aluminum oxide coatings.
It is therefore an objective to identify aqueous sealant compositions, concentrates for preparing the compositions, and processes for using the same, which are free of added cobalt or nickel and provide high quality seal, i.e. characterized by an ADT weight loss of about 3 mg./in..sup.2 or less and preferably about 2 mg./in..sup.2 or less.
It is a further object to obtain high quality seal in the substantial absence of smut formation.
It is another objective to identify aqueous sealant compositions which are effective at reduced temperatures, i.e. below the boiling point.